GDI font height >=1, fixes issue #73

The height was being rounded to 0 if scaleY was small enough, but height=0 causes the GDI font mapper to use the default height http://msdn.microsoft.com/en-us/library/windows/desktop/dd183499(v=vs.85).aspx
which caused the returned font to be larger than the allocated TileSheet, and TexField::Layout() would enter a never-ending loop of allocating new sheets, filling up the memory until ultimately crashing

Changed the calculated height to always be larger or equal to 1

Successfully tested on windows and neko targets using the code in the issue
Visually the new behavior is identical to flash target - the textfield is not visible if the scale is small enough